The invention relates to a method for treating products, which may contain cellular material of eukaryotic and or prokaryotic origin, in particular micro-organisms, located in a device comprising two electrodes onto where voltage cycles are imposed by an auxiliary electric source such that in the device and in the product electrical fields are created for a short period of time.
A prior art method of this type is known as the Pulsed Electric Field (PEF) process.
Many biological systems, such as micro-organisms, comprise a cell membrane to regulate its energy balance. Cell membranes consist of a lipid double layer whereby the lipids are made of a polar head and a fatty acid tail. Metabolic processes are regulated by said cell membrane. Physical damage of the cell membrane may lead to inactivation of the system or to an increase of the exchange of mass transport through the membrane such as inter-cellular material and/or compounds present in the bulk of the product. In the case of micro-organisms damage to the cell membrane may lead to inactivation of the organism such that the cell division process will be interrupted or its functional abilities to produce metabolic compounds is affected
Damage to the cell membrane of micro-organisms may be caused by bringing the micro-organisms into a high electric field. An sufficiently high externally imposed potential difference across the micro-organism is believed to lead to damage of the cell membrane as it leads to the inactivation of the micro-organisms as such. A treatment based on PEF can performed by using a pulsed DC voltage source. The above mentioned PEF process relies on the use of high voltage pulses to generate a pulsating electric field of in a product of such a short duration that the heating of the bulk product is restricted.
A very simple system in which the PEF-method is applied is described in U.S. Pat. Nos. 5,393,541 and 5,447,733. Both related publications illustrate a system comprising a container which is filled by product to be treated and a metal electrode which is lowered into the container. The container itself forms the other electrode and both electrodes are connected to a power supply delivering pulses of at 2 kV or more with a duration of typically two microseconds.
Another embodiment of a chamber for treating fluid products according to the PEF-method is described in U.S. Pat. Nos. 4,695,472 and 4,838,154. In this embodiment two flat electrodes are positioned opposite each other with a flow channel in between. Both electrodes are connected to a power source which during operation generates pulses. In this configuration a pulsed electrical field is produced within the product inside the channel in agreement with the PEF-method. As described, in both patents the product is subjected to high electric field pulses each having a minimum field strength of at least 5 kV/cm and each having a duration of at least about one micro-second. Preferably a duration in the range from about 5 to about 100 micro-seconds.
A further example of a system in which a PEF-method is performed is described in U.S. Pat. Nos. 5,235,905, 5,776,529 and an article with the title xe2x80x9cInactivating Micro-organisms Using a Pulsed Electric Field Continuous Treatment Systemxe2x80x9d by Bai-Lin Qin published in IEEE Transactions on Industrial Applications, Volume 34, nr. 1, 1 February 1998, pages 43/49. This prior art system comprises a socalled coaxial treatment chamber. During operation electrical pulses are supplied to both electrodes such that electrical field strengths in the range of 35 to 55 kV/cm are developed. Preferred pulse duration""s are less than 100 milliseconds, more preferably in the range of 0.1 to 100 microseconds and even more preferably in the range of approximately 0.2 to 10 micro-seconds.
A system comprising a series of tubular treatment chambers is described in U.S. Pat. No. 5,690,978. Each chamber has electrically conducting end-sections, which act as electrodes separated by a non-conducting intermediate section. During operation a pulsed electric field is developed in the treatment chamber with a typical pulse duration time of three microseconds at an applied electric field strength of E=30 kV/cm whereas the temperature reaches a maximum T=36xc2x0 C.
In all these prior art systems the medium to be treated has to be in physical and electrical contact with both the electrodes during the treatment.
A different mode of treatment is described in an article with the title xe2x80x9cInactivation of Yersinia enterocolitica Gram-Negative Bacteria using high voltage pulse techniquexe2x80x9d by Piotr, Lubicki et al published as record of the industry application conference (IAS, Orlando, Oct. 9/12, 1995, Volume 2, Number 30, pages 1338-1344, Institute of Electrical and Electronics Engineers ISBN 0-7803-3009/9, page 1339, column 1, lines 3-24). In this article a treatment device is described comprising a cylindrical electrode system including a rod shaped inner electrode inside a cylindrically shaped outer electrode. The inner electrode is connected to a source of high voltage pulses and the outer electrode is electrically grounded. The product to be treated is contained between both electrodes in a helical shaped glass tube and the remaining space within the electrode system is filled with water.
During operation a pulsed electrical field is developed between the electrodes where the rise time of each pulse is between 500 and 1300 nanoseconds and the voltage has a peak value equal to 45, 60 or even 75 kV. The article, however, does not provide any information about the electrical field strength within the product to be treated nor the processing temperatures of the product. In the article it is stressed that xe2x80x9cin order to cause electroporation of a cell membrane, the voltage magnitude must be high enough to induce suitable value of transmembrane potential for breakdown of the membrane, and at the same time, duration of the voltage pulse must be at least higher then the relaxation time of a bacteria suspensionxe2x80x9d. The product to be treated in the described model is a solution of NaCl in water for which ∈=0.7 nF/m with an electrical conductivity between 0.8 and 1.2 S/m. The relaxation time is therefore between 0.6 and 0.9 nanoseconds. In other words, the above mentioned rise time of 500 to 1300 ns is indeed significantly larger than the relaxation time of the product to be treated. It is furthermore indicated that xe2x80x9cthere is no remarkable effect of increasing rise time within the range of 500 to 1300 nsxe2x80x9d.
An objective of the invention is now to provide another method for treating suitable products, which may contain micro-organisms by developing pulsed electrical fields within the product by a different coupling. More specific it is an objective of the invention to provide a method for mild preservation of products where direct contact between the product and the electrodes is not required and where a different phenomenon is exploited to produce a substantial electrical field inside a product.
In agreement with these objectives, the invention now provides a method for treating products by bringing the product into a treatment device containing two electrodes onto which a rapidly changing high voltage difference is imposed. The two electrodes are connected to an electronic circuitry such that the device and the product are subjected to a time dependent voltage. The time dependence of the imposed voltage is primairily characterised by the rise time of the voltage which is in duration shorter than the so-called relaxation time of the product. The relaxation time has to be understood as the time necessary to obtain a complete separation of charges in a product from the moment an external voltage difference is induced over a product column. The charges in a food product may be the result of a mineral salt content of e.g. NaCl or KCl. In foodstuffs of sufficiently high water content the NaCl molecules are dissolved as Na+ and Clxe2x88x92 ions. The relaxation time can be expressed as xcfx84=∈/"sgr" whereby "sgr" is the electrical conductivity of the fluid and ∈ is the dielectric constant or permittivity.
Dynamical Polarisation Process
This method according to the invention, called the Dynamic Polarisation Process or DPP method, is based on the insight that foodstuffs and bio-mass in general are neither not very good conductors nor insulators. Typically, the electrical conductivity of high water content foodstuffs range from 0.1 S/m to 10 S/m and the permittivity is close to the permittivity of water i.e. 0.71 nF/m. As a result, a product column that is initially polarised by an external imposed voltage difference, will lose its polarisation after 0.07 to 7 nanoseconds. In this application this impulse response is exploited as follows: if an electrical voltage is imposed sufficiently fast by means of an external source, an electrical field will be present inside the product for a duration equal to the relaxation time. As soon as a pre-determined maximum peak amplitude is obtained, the external imposed voltage is allowed to vanish. Thus, it is only necessary to reach a maximum required voltage in order to induce a voltage gradient within a product for a certain period of time. The treatment can be applied several times by allowing more cycles as described previously. The level of the required voltage difference needed (or electrical field strength) in a particular application depends on the type of bulk medium, the micro-organisms under consideration and the number of cycles. Note, that micro-organisms that are present in the product will be affected by the voltages cycles as well. As the dynamical polarisation process is distinctly different from the PEF process, the interaction with organisms is of a different origin. As no charge is displaced in case of DPP and the coupling to the product is capacitive the inactivation of rigid micro structures as bacterial spores may be possible as well.
It is preferred that the described DPP method is performed under circumstances whereby the trailing edge of each pulse has ended within the relaxation time of the product. Under these circumstances any electrical current due to movement of charges is prevented even if there is a physical and or electrical contact between the electrodes and the product. In general, the DPP process can be applied e.g. in continuous flow where product is pumped through a device in which treatment takes place during its residence. Product preparation, treatment and after handling of foodstuffs, pharmaceuticals etc. can be are similar to systems where heat pasteurisation/sterilisation or PEF treatment are applied in continuous flow. The exception is that treatment is employed at DPP conditions.
In case of batch operation it is preferred that there is no direct contact between the electrodes and the product. In this case the electrodes in a treatment device can be the plates of a capacitor configuration and the product just has to be present between said plates. In such a embodiment of the method, the product is not confined to a specific tube, channel etc. defined by the treatment device itself but the product is present e.g. within a suitable package which is placed into the device. Examples of products that may be treated in batch are pouches, boxes, containers but also complete eggs in shell. These products can be treated in a (semi-) continuously fashion. For this an controlled automated system may be used comprising a conveyor belt and a treatment device which is connected to suitable electronic source to apply the DPP method.